We have some API in GtkMenuTracker and GtkMenuTrackerItem that is
specifically designed to deal with submenus.
Generalise these APIs to take a 'link_name' parameter that we always
give as G_MENU_SUBMENU for now. In the future, this will allow creating
trackers for other types of links, such as sections.

Allow the possibility for items to be marked with a special attribute and
expose this via GtkTrackerMenuItem. For internal use only.
We will use this to implement the special 'Hide', 'Hide Others' and 'Show All'
items and the 'Services' submenu in the Mac OS application menu.
https://bugzilla.gnome.org/show_bug.cgi?id=720552

Add an internal API for checking if a GtkMenuTrackerItem is visible,
along with a signal for reporting changes in that flag. The item will
become invisible in situations according to the new hidden-when=''
attribute, which can be set to 'action-disabled' or 'action-missing'.
This new flag doesn't actually do anything yet, and none of the
consumers of GtkMenuTracker do anything with it (nor should they). A
followup patch will address the issue.
https://bugzilla.gnome.org/show_bug.cgi?id=688421

Strictly speaking, can_activate should always be set back to FALSE when
the action disappears from the muxer (since we can't activate it
anymore) but we forgot to do that.
This 'bug' could never cause a problem because 'can_activate' is never
directly queried for anything at all and the item would get marked
insensitive anyway. As soon as the action was re-added, can_activate
would be recalculated based on the new action before anything else could
happen.
All the same, this should be cleared here.
https://bugzilla.gnome.org/show_bug.cgi?id=688421

Add support for pulling the primary accel out of the GtkActionMuxer.
With this change, it is no longer necessary to have the accel=''
attribute hardcoded onto each menu item (and, in fact, it should be left
off if you intend to have support for dynamic accelerator changing).
Specifying accel='' is a good way to force an accelerator not to be
displayed on a menu item.

Store "action and target" format inside each GtkMenuTrackerItem. This
makes action invocation more efficient (no hash table lookups or
allocations) and slightly simplifies handling of action namespace.
More importantly, this will be used when we start to get accels from
GtkActionMuxer.

Add a new class, GtkMenuTrackerItem that represents a menu item, to be
used with GtkMenuTracker.
GtkMenuTracker's insert callback now works in terms of this new type
(instead of passing reference to the model and an index to the item).
GtkMenuShell now handles all of the binding tasks internally, mostly
through the use of property bindings. Having bindings for the label and
visibility attributes, in partiular, will help with supporting upcoming
extensions to GMenuModel.
GtkModelMenu has been reduced to a helper class that has nothing to do
with GMenuModel. It represents something closer to an "ideal" API for
GtkMenuItem if we didn't have compatibility concerns (eg: not emitting
"activate" when setting toggle state, no separate subclasses per menu
item type, supporting icons, etc.) Improvements to GtkMenuItem could
eventually shrink the size of this class or remove the need for it
entirely.
Some GtkActionHelper functionality has been duplicated in
GtkMenuTracker, which is suboptimal. The duplication exists so that
other codebases (such as Unity and gnome-shell) can reuse the
GtkMenuTracker code, whereas GtkActionHelper is very much tied to
GtkWidget. Supporting binding arbitrary GtkWidgets to actions vs.
supporting the full range of GMenuModel features for menu items turns
out to be two overlapping but not entirely similar problems. Some of
the duplication (such as roles) can be removed from GtkActionHelper once
Gtk's internal Mac OS menubar support is ported to GtkMenuTracker.
The intent to reuse the code outside of Gtk is also the reason for the
unusual treatment of the enum type introduced in this comment.
This adds no new "public" API to the Gtk library, other than types that
we cannot make private due to GType limitations.